Cellulose is one of the most abundant carbonaceous materials available on earth; some billion tons of cellulose are being formed annually by the natural process of photosynthesis. Efficient and economic conversion of cellulose to compounds such as ethanol which can serve as fuels and chemical feedstocks could significantly reduce our dependence on non-renewable, hydrocarbon-derived resources.
Gauss et al., U.S. Pat. No. 3,990,944, and others, have demonstrated that biological conversion of cellulose using microorganisms and/or enzymes to ethanol or other chemicals is a specific and expeditious method of utilizing this resource. However, no such biological process has yet reached large-scale commercial practice due to the uncertain economic feasibility of these operations. The invention described herein provides a substantial improvement over the prior art, which considerably reduces the cost of converting cellulose to ethanol or other chemicals.
In any biological process for cellulose conversion, a cellulose-containing feedstock such as wood processing wastes, sugarcane bagasse, pulping wastes, rice straw, or hydropulped municipal solid waste is contacted with a source of cellulolytic enzymes. The feedstock is generally pretreated by some physical, mechanical, chemical, thermal, or combination process to improve the access of the cellulolytic enzymes to the cellulose. This improvement occurs through particle size reduction and liberation of the cellulose from lignin and other feedstock components which hinder the access to enzyme. Previous investigations, e.g. those of Gum and Brown, Biochem. Biophy. Acta, 446, 370-386 (1976), have shown that the cellulase enzyme complex elaborated by the mold Trichoderma reesei QM 9414 contains a multiple of enzymes, e.g. an endoglucanase (E.C.3.2.1.4), and cellobiohydrolase (E.C.3.2.1.91) and a beta-glucosidase (E.C.3.2.1.21), needed for the rapid hydrolysis of cellulose to simple sugars such as cellobiose and eventually glucose. The formed and released soluble sugars can be fermented by yeasts such as Saccharomyces cerevisiae or Candida brassicae or bacteria such as Zymomonas mobilis or molds such as Rhizopus javanicus to ethanol.
It will be noted in the system described above there are four essential components:
1. A cellulose feedstock.
2. A source of cellulase enzyme complex.
3. Simple sugars such as glucose released by the action of cellulases on cellulose.
4. Microorganism capable of fermenting sugars to ethanol.
It is known by those skilled in that art that during the enzymatic hydrolysis of cellulose to sugars, the rate of the hydrolysis reaction can be restricted by several factors; for example, addition of a larger quantity or more potent enzyme to a given amount of cellulosic feedstock will generally increase the hydrolysis rate. However, as additional quantities of enzyme are added to the cellulose, less positive and then negative effects on the hydrolysis rate are observed, as shown in FIG. 1, curve A. This is most likely the result of the saturation of the active surface sites on the cellulose-containing particles by the surplus enzyme, whereupon addition of further enzyme has no beneficial effect. The rate of reaction is also limited by the concentration of cellulosic feedstock which can be suspended in the aqueous enzyme-containing reaction slurry. Above a certain concentration, the value of which depends on the specific feedstock, the viscosity of the liquid becomes so high that a plastic or semi-solid consistency is assumed, and pumping or mixing becomes difficult. It has been shown to be beneficial to add certain polymeric surface-active materials to the reactor during enzymatic hydrolysis to cellulose. Moo-Young, U.S. Pat. No. 3,975,236, discloses that materials such as high molecular weight carboxypolymethylenes can increase the production of cellulolytic enzymes during the growth of Trichoderma species on cellulose.